The present invention relates in general to a sputtering cathode apparatus used for the deposition of thin films. More particularly, the invention is concerned with an improved sputtering cathode construction that provides for deposition of uniform films at high deposition rates.
Planar magnetron sputtering is commonly used for the deposition of thin films in both laboratory and production applications. Planar magnetron sputtering permits relatively high deposition rates, limits unwanted substrate bombardment and heating, and is particularly useful for deposition with large flat substrates without requiring planetary tooling or excessively large throw distances. In planar megnetron sputtering, there is established a visible plasma area magnetically confined to a selected region of the target which then becomes the only area with significant emission of sputtered material. The emitting area assumes the form of a closed figure in the target plane.
Economic considerations dictate optimization of sputtering throughput in deposition systems and apparatus. An increase in effective deposition rate is translatable into increased system productivity and lower cost per unit area of thin films. Optimally achievable rates are limited basically by two factors. The first factor relates to target power loading and the second factor relates to instantaneous deposition rate. With regard to target power loading, the RF or DC power supplied per unit area to the emitting regions of the target is limited by considerations of target integrity. Excessive power loading causes melting or fracture of the target or leads to failure of target bonding. With respect to instantaneous deposition rate, for substrate areas directly over emitting target areas, the instantaneous deposition rate may reach levels inconsistent with desired film morphology, or can lead to excessive substrate heating. Exessive substrate particle bombardment may also occur.
In presently employed cathode sputtering systems or apparatus, the substrate upon which the thin film is to be deposited moves past the target to insure deposition uniformity. In the course of one cycle of linear or rotational motion, each area of the substrate is exposed to substantially identical deposition conditions, on average, leading to uniformity in film thickness and material parameters. A typical prior art arrangement is depicted in the schematic view of FIG. 1. FIG. 1 shows a rectangular target 10 with an emissive plasma region 12 in the shape of a rectangular-shaped picture frame. FIG. 1 also illustrates the rectangular substrate 14 upon which the thin film is being deposited. In FIG. 1 there are shown two positions of the substrate 14. The substrate is shown in solid at position A and is shown in dotted at position B. Substrate motion may be in a single sweep from left to right or from position A to position B. Alternatively, there may also be provided a return pass from position B to position A. Alternatively, multiple passes with many complete cycles may also be used. As the substrate makes its pass relative to the target 10, significant emission of source material occurs only from the plasma area 12. In this regard, it is noted that there are two long legs 16A and 16B, each of which has a width L. These two legs contribute almost all of the material that is being deposited since the substrate does not pass over the shorter end regions.
In the prior art construction illustrated in FIG. 1, it is noted that any given area on the substrate spends most of the deposition time in regions not over the plasma area 12. Thus, the time when significant quantities of sputtered material are received at this given area of the substrate is relatively short in duration compared with the time required for transit from say position A to position B.
The ratio of effective to maximum instantaneous deposition rate may be expressed by a geometric efficiency factor g which is defined as the ratio of the time spent over the plasma emission area (area 12 in FIG. 1) to the total process time. For low values of efficiency factor g, the product throughput is low even for maximum allowable target power. The value of geometric efficiency factor g for the system depicted in FIG. 1 is estimated by computing the ratio of times for a single pass at a particular speed v. Actually, the result is independent of the speed v and the number of passes in a run.
In transit from position A to position B at say speed v, a given small area on the substrate takes time L/v to cross one plasma leg, and time 2 L/v to cross both legs. The total time over the plasma is thus expressed by the ratio 2 L/v. The total time taken for a pass is (2a+t+d)/v, where a is the minimum clearance required for uniform sputtering, t is the target width, and d is the substrate width. Each of these parameters is illustrated in FIG. 1.
The geometric efficiency factor is thus g=2 L/(2+t+d). For a typical system, we can set some representative values such as L=1", a=4", t=5" and d=12". By inserting these values into the formula, the geometric efficiency is g=0.080. Thus, the effective deposition rate with this prior art arrangement of FIG. 1 is only 8% of the possible maximum instantaneous rate.
Moreover, with the arrangement of FIG. 1, the necessary size of the system is not at all optimized. This thus requires an excessively large vacuum system for the sputtering operation. The critical dimension in this regard shown in FIG. 1 is the dimension W. In the example previously given, a minimum horizontal dimension W of the system of FIG. 1 is the sum of 2a+2d+t. In the example previously given, this calculates at W=37".